1. Field of the Invention
The present invention relates to a developer supply system or particularly to an improvement of the developer supply system employing a process for feeding in a horizontal direction a developer by the rotations of a spiral, screw roller, etc. from the developer supply side of a supply opening to the terminal side until the developer falls down into a developing unit.
2. Description of the Prior Art
A conventional developer supply system 11 will be described with reference to FIGS. 3-5. FIG. 3 uses two reference numbers 1 and 11 to identify the developer supply system because it is in common with the developer supply system 1 in accordance with the present invention.
The developer supply system 11 is mounted on a developing unit 10. Said developing unit 10 has a developer sensor 3 for detecting the concentration of developer (concentration of toner in case of use of a developer of two-component system). If the concentration of developer is lowered, a spiral 4 is rotated by means of a drive mechanism 7. When the spiral 4 is put in motion, a developer contained within a developer supply tank 2 is transported in a horizontal direction in a pipe 5 and fallen from a supply slit 6 (supply opening) into the developing unit 10. For the purpose of having a uniform distribution of the developer in drop rate, a portion of the supply slit 6 near the supply side for the developer is formed at such a high level that when piled up considerably high within the pipe 5, the developer will fall down, while another portion of the supply slit 6 near the terminal side (opposite to the supply side) is formed at a low level so that the developer may fall down even if the developer within the pipe 5 has become low due to supply of the developer to the developing unit.
The developer sensor 3 is provided in a position corresponding to the center of the supply slit 6. This arrangement makes it possible to detect decreasing of the developer at the center because the developer tends to be consumed to the greatest degree at this point when smallsized objects to be copied are subjected to repeated copying operations.
FIGS. 5(a)-(d) are views showing the change of the conventional developer supply system 11 in the operational phase with time.
FIG. 5(a) shows an initial state of operation wherein the developer T has started to be supplied from the developer supply tank 2 to the supply slit 6. Dropping of developer T into the developer unit 10 is only limited to a portion where the developer T has reached the supply slit 6. Further continuous rotation of the spiral 4 allows the developer T to arrive at the other side of the supply slit 6, which state is shown by FIG. 5(b).
In this state of FIG. 5(b), the developer T has been distributed substantially uniform in the range between the end of one side (supply) and the end of the other side (terminal) of the supply slit 6. The driving of the spiral 4 is controlled in response to a signal from the developer sensor 3. If the concentration of developer is decreased, the spiral 4 is activated to supply the developer from the developer supply tank 2 until the developer falls from the supply slit 6. And if the concentration of developer is sufficiently increased, the spiral 4 is stopped and the supply of the developer is discontinued. This will establish a balance between the drop of developer from the supply slit 6 and the supply of developer from the developer supply tank 2, and the height of the developer within the pipe 5 may be maintained at a determined level.
As shown by FIG. 5(c), however, when the developer supply tank 2 is exhausted, thereby resulting in a discontinuation of supply of the developer T to the pipe 5, the developer T is too reduced in quantity to continue to dropping at the area near the end of the supply side of the supply slit 6, and then, the dropping of the developer will take place only at the area near the end of the terminal side. And when the area where insufficient amount of developer remains spreads over to a position corresponding to the center of the supply slit 6, the developer sensor 3 acts to detect it, and then the spiral 4 is actuated to deliver a fresh developer. However, since a rotation of the spiral 4 fails to overcome the deterioration of the developer in concentration, it will be determined that the developer supply tank 2 has been exhausted. Consequently, it is impossible to detect the developer supply tank 2 being emptied until the area with no sufficient developer left spreads from the end of the supply side of the supply slit 6 over to the center. Namely, there lies a problem that the arrangement of the prior art involves a time delay in determining that the developer supply tank 2 no longer contains a developer. Further problem is that there may create non-uniformity in concentration of developer that the specified concentration of developer fails to remain, rather being inclined downward at the area of the developing unit 10 close to the supply side of the supply slit 6, whereas at the other area of the developing exist unit 10 close to the terminal side increasing of the concentration of developer exist.
There is another problem that even though the developer T.sub.2 is added to the empty developer supply tank 2 after the detection of its emptiness, as shown by FIG. 5(d), a portion through which no developer drops may occur between the portion from which the old developer T.sub.1 drops upon delivered to the terminal side of the supply slit 6 and the portion through which the developer T.sub.2 drops after reached the end of the supply side of the supply slit 6, whereby some time is required until the uniform distribution of developer in drop rate is obtained again.